Slow spi_sync() on pxa2xx_spi

[Stefan Schmidt]

Hello.

On Thu, 2011-06-16 at 17:44, Eric Miao wrote:
> On Thu, Jun 16, 2011 at 5:19 PM, Stefan Schmidt
> <stefan at datenfreihafen.org> wrote:
> >
> > Its either 8bit or 16bit for a message. The mentioned problem happens
> > on a 8 bit message. It does nothing else then writing to a defined
> > register address to trigger the hardware sending the data in the FIFO.
> 
> This might get too much overhead. I didn't check the source code, but
> my guess is it's waiting for interrupt for each message, and once the
> interrupt of message being flushed out happens, it starts the next one
> waiting for the FIFO empty interrupt coming. Did you try stuff more
> bytes into a single message?

Took me some time to implement and test al kind of different
scenarios. As you suggested I tried to stuff more bytes into a message
as well as testing all kind of combination with tx and rx thresholds,
timout, PIO and DMA, cs_change , etc. No change.

I then bit the bullet and changed the board to use the spi_gpio driver
instead of pxa2xx. After some struggling (make sure you setup pin
config correctly :)) I got it working but with the same result.

My last test block was around using spi_async() instead of spi_sync
and as well I was not able to reach my timing criteria. :(

To me it seems right now as if it is just not possible to reach
something in the 100 usec range with the SPI framework. Obviuously I
would be loved to be proven wrong. :)

As I'm running way out of time for this part of my project I'm back to
the driver now trying to find a solution that avoids the tight timing
criteria.

Eric, thanks again for your fast and helpfull mails.

regards
Stefan Schmidt